Photoluminescence (PL) spectroscopy is an essential tool to study photo-excited electronic processes [Lakowicz, Joseph R., ed. Principles of fluorescence spectroscopy. Springer, 2009.]. Among various PL spectroscopy techniques, time-resolved photoluminescence (TRPL) spectroscopy has been shown to be a powerful tool to study photochemistry, photophysics and photobiology [Fleming, Graham. Chemical applications of ultrafast spectroscopy, (1986). Beechem, Joseph M., and Ludwig Brand, Annual review of biochemistry 54.1 (1985): 43-71.]. The advantages of TRPL include high sensitivity and selectivity. With careful experimental design, TRPL can be performed as a background free measurement to probe the weak interaction of the excitations. Because TRPL selectively probes emissive photoexcitations, it also provides valuable information to study complex systems that involve various excited species. In contrast to other time resolved spectroscopy techniques which are based on complex nonlinear processes, TRPL only evolves with linear interaction process between the photon and the sample. Thus, it can provide rich information about the dynamics of excited states, and the signal interpretation is straight forward.
The TRPL techniques can be classified by two classes: pulse fluorometry and phase-modulation fluorometry [Lakowicz, Joseph R., ed. Principles of fluorescence spectroscopy. Springer, 2009.]. The present disclosure is focused on the pulse fluorometry method. In pulse fluorometry, the sample is excited by a short laser/optical pulse. Then, the PL signal is gated by a fast optical shutter and measured as function of time. The PL can be gated electronically or optically. TRPL data contains two dimensions; the spectral domain and the time domain, each of which needs to be accounted for when specifying the performance of a TRPL system, in addition to the sensitivity and signal to noise. In the time domain, the most important parameter is the time resolution. For the electronic gating method which is based on fast response electronics, the time resolutions range from sub-nano to nanosecond. Two of the most widely use electronic gating methods are time-correlated single photon counting (TCSPC) [Y. V. Il'ichev, W. Kuhnle, and K. A. Zachariasse, J. Phys. Chem. A 102, 5670 (1998); M. P. Heitz and M. Maroncelli, J. Phys. Chem. A 101, 5852 (1997).] and streak cameras [Campillo, A., and S. Shapiro. Quantum Electronics, IEEE Journal of 19.4 (1983): 585-603; B. Gobets, I. H. M. Van Stokkum, M. Rogner, J. Kruip, E. Schlod-der, N. V. Karapetyan, P. Dekker, and R. Van Grondelle, Biophys. J. 81, 407 (2001).]. Although new generation streak cameras can have sub-picosecond time resolution, they suffer from low sensitivity and picosecond timing jitter causes difficulty in synchronizing the camera and the excitation laser. In the spectral domain, the most important parameter is the spectral bandwidth. The TCSPC is a single channel detection technique. This means that it needs several scans for different wavelengths to reconstruct the full time resolved spectrum. The streak camera can use a two dimensional detector as the sensor for broadband detection.
Although TRPL techniques with sub-nano to nanosecond time resolution have become standard tools in various fields, recent research interests have shifted to the ultrafast dynamics in sub-pico to picosecond time scale [Qiu, Weihong, et al. Proceedings of the National Academy of Sciences 104.13 (2007): 5366-5371; Messina, Fabrizio, et al Nature communications 4 (2013); Banerji, Natalie. J. Mater. Chem. C1.18 (2013): 3052-3066.]. The application of the ultrafast TRPL includes the ultrafast solvation dynamics [Jimenez, Ralph, et al. “Femtosecond solvation dynamics of water.” Nature 369.6480 (1994): 471-473.], energy transfer [Klostermeier, Dagmar, and David P. Millar. Biopolymers 61.3 (2002): 159-179.] and charge transfer [Messina, Fabrizio, et al. Nature communications 4 (2013).] processes. Valuable information can be extracted from the spectra and the spectral evolution on the ultrafast time scale. Therefore, ultrafast broadband TRPL techniques are an important tool for both fundamental and applied research. Until now, it is still impossible to catch such short events with electronic gating methods. Modern ultrafast lasers and optical gate based on nonlinear optical process provide the solution for ultrashort time resolution. Femtosecond fluorescence up-conversion [Shah, Jagdeep, Quantum Electronics, IEEE Journal of 24.2 (1988): 276-288.] is the most widely used method for the optical gating TRPL. By using the fundamental output (800 nm) of the commercially available Ti-Sapphire laser system as the light source, the up-conversion system can perform with high sensitivity and sub-picosecond time resolution. However, the disadvantage of up-conversion TRPL is that the detection bandwidth is limited by the narrow phase matching bandwidth of the second order sum frequency process. Thus, it is difficult and time consuming to get the ultrafast spectra by the up-conversion system. It is possible to achieve broader phase matching for the up-conversion process by selecting special pumping wavelength nonlinear crystal and noncollinear phase matching [Zhang, X. X., C. Wurth, et al. (2011), Review of Scientific Instruments 82(6): 063108-063108.]. However, the design and implementation of the setup is complicated.
An alternative way to realize broadband ultrafast TRPL is the optical Kerr gate [Nakamura, R. and Y. Kanematsu (2004), 75(3): 636-644; Arzhantsev, S. and M. Maroncelli, 2005, Appl. Spectrosc. 59(2): 206-220.]. The ultrafast optical shutter is constructed by the Kerr medium and a pair of high quality polarizers and controlled by the optical Kerr effect induced by the ultrafast laser pulse. Theoretically, because of the inherent phase matching condition of the Kerr effect, the Kerr gate is an ideal design for the broadband ultrafast TRPL. Practically, its performance is limited by the useful bandwidth, and the transmission and extinction ratio of the polarizers. In the Kerr gate setup, because the un-gated PL and gated signal are collinear, a polarizer pair is needed with a high extinction ratio to block the un-gated PL. However, such polarizers are still unavailable, especially in the UV range, and the application of the Kerr gate system is limited to samples with short PL lifetimes and low quantum yields due to the difficulty in suppressing background PL.
Transient gratings (TGs) produced by the Kerr effect from laser pulses have been proposed for optical deflection schemes. See Alfano et al., U.S. Pat. No. 5,126,874 issued Jun. 30, 1992. Such optical deflection schemes have been limited by the relatively small amount of beam energy that is deflected compared to the incident or undeflected beam energy and compared to the beam energy required to create the transient grating. The dynamic TG response of a material can be used to study its photoexcitation dynamics [H J Eichler, P. Gunter, and DW Pohl, Laser-Induced Dynamic Gratings (Springer-Verlag, Berlin, 1986)]. Also, Lee et al. used a TG method to measure broadband ultrafast supercontinuum pulses [Lee, D., Gabolde, P., & Trebino, R. (2008), Journal of the Optical Society of America B, 25(6), A34].